Originally Posted by
spornrad
A third possible reason could have been minimizing HW changes for a last minute, rushed band aid. MCAS is basically a software module running on the pre-existing FCC. HW consists only of two additional relays, one for "column cutout override" and one for "flaps up high trim motor speed", plus some cabling.
Only guessing as well, of course. As usual with most last minute changes under pressure, it is an exceptionally bad idea to abuse speed trim as a stick pusher.
That was done with the first, high Mach, limited authority, version of MCAS. It was not rushed. It was found in wind tunnel testing. Multiple options were tested before MCAS was developed.
The last minute aspect (correction for issue discovered in flight test) was software only, adding the low Mach, higher authority operation.